(1) Field of the Invention
The present invention relates to a magnetic brush development method. More particularly, the present invention relates to a magnetic brush development method for forming a toner image excellent in the image characteristics by using a two-component type developer comprising a ferrite carrier and electroscopic toner particles.
(2) Description of the Prior Art
In the electrophotographic process using a two-component type magnetic developer, an electroscopic toner is mixed with a magnetic carrier, this two-component type composition is supplied onto a developing sleeve having a magnet installed therein to form a magnetic brush consisting of this composition, and this magnetic brush is brought into sliding contact with an electrophotographic photosensitive plate having an electrostatic latent image formed thereon, whereby an electroscopic toner image is formed on the photosensitive plate. The electroscopic toner is charged with a polarity reverse to that of the polarity of the charge of the electrostatic latent image on the photosensitive plate by the friction with the magnetic carrier, the electroscopic toner particles on the magnetic brush are attracted onto the electrostatic latent image by the Coulomb force to effect the development of the electrostatic latent image. On the other hand, the magnetic carrier is attracted by the magnet in the sleeve, and since the charge polarity of the magnetic carrier is the same as that of the electrostatic latent image, the magnetic carrier is left on the sleeve. In order to form a clear image having a high density, it is important that a sufficient difference of the relative speed should be produced between the photosensitive plate and the magnetic brush so as to bring about sufficient sliding contact between the photosensitive plate and the magnetic brush.
An iron powder carrier is ordinarily used as the magnetic carrier. However, this iron powder carrier is still insufficient in various points. For example, a two-component type developer comprising this iron powder carrier is defective in that the rising of the development sensitivity curve (the curve showing the relation of the difference of the voltage between the electrostatic latent image and the developing sleeve to the image density) is sharp, and the gradation characteristic is poor and the reproducibility of a halftone is insufficient. Furthermore, a developer comprising this iron powder carrier often forms a hard magnetic brush and involves a risk of impairing the photosensitive layer, and at the reproduction of a solid black portion, brush marks, that is, many fine and short white lines extending in the sliding direction of the brush, are formed on the resulting image. Moreover, the iron powder carrier is sensitive to the humidity and the development characteristics are changed according to the humidity, or rusting is caused in the iron powder carrier. Still further, this iron powder carrier is defective in that a large torque is necessary for driving the magnetic brush.
Use of ferrite, especially soft ferrite, as the magnetic carrier of a two-component type developer has recently been proposed. However, since the ferrite carrier has an electric resistance higher than that of the iron powder carrier, such troubles as carrier drawing, that is, migration of the carrier to the photosensitive layer, and occurrence of edge effects in the formed image are readily caused.
In the magnetic brush development using a ferrite carrier, phenomena not observed in the development using an iron powder carrier are thus caused to occur. However, the conditions for this magnetic brush developing have not been sufficiently elucidated.